The present invention generally relates to piston seals and more particularly relates to a sealing insert.
In motor vehicles which, for example, are equipped with an antiblock system (ABS) or traction control system (TCS), a so-called tandem master cylinder with a central valve usually is an essential part of the hydraulic brake system. When such a tandem master cylinder with a central valve is used, an associated sealing (primary sealing member) is not damaged when brake pressure is built up suddenly due to ABS or TCS operations; contrary to master cylinders with radially oriented supply bores, so-called breathering holes.
In general the central valve in the cavity of the associated piston assumes the same function as the supply bore together with the primary sealing member, i.e. it ensures the required equalization of quantity and pressure of hydraulic brake fluid between the master cylinder and the brake lines connected thereto within a dual-circuit brake system. In other words the equalizing port is not needed, since its function is assumed by the central valve. The sealing insert disclosed hereunder regularly seals off one end of the housing of the tandem master cylinder, holding and sealing the piston, which can be activated in this way, in an axially displaceable manner in an associated hole enclosed by the master cylinder.
Such sealing inserts usually comprise a supporting body and rubber-elastic sealing body that is adhesively connected to the supporting body. For this purpose, said sealing body can be sprayed or vulcanized on the supporting body (please refer to DE-OS 39 32 248).
A similar process is used for a sealing insert of the kind described above, where the sealing body is vulcanized or cast on the supporting body. In addition, a ring-shaped collar is definitely necessary (see DE-OS 42 15 079) for the supporting body in this disclosure so as to improve the absorption of axial forces between the associated sealing body and supporting body.
Finally, in another embodiment the sealing body and supporting body are connected to one another in a separable way, and a ring-shaped circumferential projecting part that forms one piece with the supporting body is provided, with such projecting part axially extending through the sealing body completely or nearly completely. In this way, it is ensured that the central valve opens at defined intervals (see DE-PS 196 10 834).
The above-mentioned state-of-the-art is not completely free of deficiencies. Thus, perfect operation of the central valve is achieved by connecting the sealing body and supporting body separably; as a consequence, there are disadvantages as far as storage and assembly are concerned. Usually one strives for a minimum number of parts for series production in automobile manufacturing (see DE-PS 196 10 834). When studying the other embodiments, it needs to be pointed out that a form-fit and perfect connection between the sealing body and supporting body is not always ensured for the entire life despite such additionally realized measures as the ring-shaped collar. It is here that the present invention proposes a solution.
The present invention is based on the technical problem of designing further a sealing insert of the kind described above, so that, firstly, the sealing function is perfect when the central valve operates with defined valve timing and, secondly, the component can be produced simply and cost-efficiently, using as few parts as possible, and with a long service life.
The solution proposed by this invention is that the supporting body of a generic sealing insert exhibit through holes which are penetrated by the sealing body for fixing purposes. Contrary to the usual ring-shaped supporting body, these through holes are generally arranged radially.
In other words, the supporting body basically is designed as a ring with radial through holes, i.e. the through holes completely extend through the supporting body when viewing the cross-section. Naturally, other embodiments of the supporting body (i.e. no ring shapes) are conceivable within the scope of the present invention, even if this is not customary because the associated piston as well as the housing of the master cylinder generally are cylindrical.
In order to ensure that the through holes can be produced in a particularly simple way, through holes arranged opposite one another have parallel areas of contact. In this way, the through holes can be inserted in the supporting body practically at the same time.
In its cross-section the supporting body generally is formed like an upside-down T with a T-bridge and T-legs; and the T-bridge usually faces the central valve, whereas the T-leg faces away from the central valve. The through holes can penetrate the T-bridge, so that the supporting body can be fixed onto the T-leg while they are being produced. In so far as the through holes are not produced subsequently and the supporting body is a (plastic) injection-molded part, it should be pointed out that in this embodiment of the invention only the (more narrow) T-bridge exhibits the through holes; hence, the overall stability of the supporting body is not affected. This is of special importance since temperature-resistant duroplastics usually are used for producing the supporting body.
As a rule, the sealing body""s cross-section is formed like an upside-down U with U-legs arranged on both sides of the T-bridge and a U-base on the face of the T-bridge; i.e. the sealing body envelopes, as it were, the T-bridge and its ends rest on each of the U-legs of the U-leg. This already ensures a large fixing area between sealing body and supporting body in the course of injection molding with a rubber-elastic sealing substance (e.g. EPDM). Naturally, customary processes, if necessary by adding bonding agents, can be applied.
The thickness of the U-base of the sealing body should correspond to one-half to one-fifth, preferably about one-third, of the thickness of the U-legs, so that the T-bridge almost completely penetrates the sealing body at this point (i.e. in the area of the U-base of the sealing body), allowing only limited elastic deformation of the U-base. In other words, according to a preferred embodiment, the U-base covers the T-bridge of the supporting body like a thin skin, so to speak. As a result the same effect as that according to DE-PS 196 10 834 is achieved, namely that the central valve or an associated stop ring can abut against the sealing insert softly and slightly dampened. At the same time, the rubber-elastic U-base is deformed only slightly due to its negligible thickness, so that defined opening of the central valve is ensured irrespective of the pressure, against which the central valve has to be opened.
In this way, a rigid or quasi-rigid end stop is provided for the central valve, without having to prejudice the achievable tightness. Such defined opening of the central valve or reliable and constant timing of the central valve is of special importance in the present invention because particularly in brake systems with a brake-pressure control system, as for example ABS or TCS, the central valve may have to be opened against considerable overpressure, especially if, for example, an ABS braking operation is necessary directly after a traction-slip control cycle. As a matter of fact, in such a case the brake-fluid pressure built up in the pressure chamber of the master cylinder or tandem master cylinder within the scope of the traction-slip control cannot be relieved prior to the braking operation following directly afterwards, because the fluid volume or brake-fluid volume supplied during the above-mentioned control cycle cannot be returned to an associated fluid tank. Depending on how high the predominant fluid pressure still is, the opening time of the central valve varies since it (via the end stop penetrates deeper into the sealing body of the sealing insert the higher the predominating pressure is. Consequently, a clear definition of the opening time or timing of the central valve is required to ensure constant brake response behavior. This is achieved within the scope of the present invention by the U-base of the sealing body, which is designed, as it were, as a thin skin as well as the T-bridge extending below the U-base, which serves as an end stop for the central valve or associated stop ring.
According to another preferred embodiment of the present invention, cross holes that communicate with the through holes are provided. In this connection, at least one cross hole is connected to a through hole, whereby the cross holes can be concentrically distributed in the T-leg. For the purpose of injection, the liquid rubber-elastic material (e.g. EPDM) can be filled into the supporting body under pressure at the same time via the cross holes and is distributed as desired via the through holes by forming the U-shaped sealing body in the cross-section, whose outside contours are defined by a corresponding (injection-molding) form. In order to ensure as great an overall rigidity (against deformation) of the sealing insert as possible, the through holes and cross holes are arranged at axially offset positions in the supporting body, i.e. they do not overlap in the direction of the (rotationally symmetrical) axis of the sealing insert.
Finally, the U-leg of the sealing body facing a (insert) hole may exhibit a V-shaped hole seal lip formed on said leg, which is supported on the T-side. Also the side of the sealing body facing away from the (insert) hole preferably exhibits a V-shaped piston seal lip formed on it, which is also supported on the T-leg.
As a result the scope of the present invention at first achieves reliable tightness by means of the injection-molded sealing insert and sealing body. The timing of the central valve practically is not affected, irrespective of the predominating fluid pressure, against which it has to be opened; hence, constant brake response behavior can be expected. At the same time, simple and low-cost manufacturing is possible, because time-proven and cost-efficient production processes as well as low-cost materials can be used. Furthermore, assembly is simplified because only one compact component is provided, which, in addition, can be mounted automatically. Moreover, a particularly firm form-fit connection between the two parts is attained by fixing the sealing body in the supporting body by means of the through holes. This has a positive effect on the service life of the valve sealing insert. These are the essential advantages of the present invention.